Threaded joints are widely used in connecting steel pipes such as oil well pipes, which are used in oil production facilities. To connect steel pipes for use in oil or gas prospecting and production, typical standard threaded joints based on the American Petroleum Institute (API) standard have been conventionally used. However, because crude oil wells and natural gas wells have recently increased in depth, and horizontal wells and directional wells have increased more than vertical wells, excavation and production environments are becoming severe. Furthermore, an increase in well development under hostile environments such as the sea and polar regions, diversifies performance requirements for threaded joints such as compression resistance, bending resistance, and external-pressure sealing performance (external pressure resistance). Therefore, this trend increases the use of special threaded joints having high performance which are called premium joints, and the demand to further improve the performance of such joints is increasing.
A premium joint is generally a coupling-type joint in which a pair of externally threaded male members (hereinafter referred to as pins) formed at ends of pipes and an internally threaded female member (hereinafter referred to as a box) are coupled. Each of the pins includes a tapered thread, a seal portion (specifically, a metal-to-metal seal portion), and a shoulder portion (specifically, a torque shoulder portion). The box connects the pins to each other. The tapered thread is important to strongly fix the pipe joint. The seal portion ensures sealability because the box and the pins come into metal-to-metal contact with each other at this portion. The shoulder portion serves as an abutment when the joint is tightened.
FIGS. 3(a)-3(c) are schematic explanatory diagrams of a premium joint for oil well pipes, which are longitudinal cross-sectional views of a threaded joint for cylindrical pipes. FIG. 3(a) is an overall view, FIG. 3(b) is an enlarged view of threaded portions (REGION B1) in FIG. 3(a), and FIG. 3(c) is an enlarged view of a region near a pin nose (REGION B2) in FIG. 3(a). The threaded joint includes pins 3 and a box 1 corresponding to the pins 3. Each of the pins 3 includes an externally threaded male portion 7 formed on the outer peripheral face thereof and a nose portion 8 (pin nose 8). The pin nose 8 is an unthreaded portion disposed at the end of the pin 3 to be adjacent to the externally threaded male portion 7. The pin nose 8 includes a seal portion 11 on the outer peripheral face thereof and a torque shoulder portion 12 on the end face thereof. The box 1, which faces the pin 3, has an internally threaded female portion 5, a seal portion 13, and a shoulder portion 14 on the inner face thereof, which are respectively capable of mating with or contacting the externally threaded male portion 7, the seal portion 11, and the torque shoulder portion 12 of the pin 3.
Threaded joints that are particularly resistant to an external pressure load have been proposed. For example, Japanese Patent No. 4535064 describes a radial-seal-type threaded joint in which a pin nose adjacent to the end of a pin comes into metal-to-metal contact with the inner face of a box in a radial direction, and the contact portion serves as a seal portion. The pin nose that is discontinuous with the seal portion (discontinuous in that the outer peripheral face of the pin nose is outside of an extension line of a seal face in the radial direction) is disposed between the seal portion and the shoulder portion, and the pin nose does not contact the box. In this threaded joint, since the seal portion is disposed to be separated from the end of the pin, high sealability can be obtained even if the thickness of the pin is relatively small.
However, the threaded joint described in Japanese Patent No. 4535064 has a problem in that the shape of the pin becomes complex and the productivity decreases, because it is necessary to form the pin nose to be discontinuous with the seal portion.